1. Field of the Invention
This invention relates to current sources formed on an integrated-circuit (IC) chip. More particularly, this invention relates to MOS-cascoded bipolar current sources such as may be used as part of a high-performance amplifier circuit. The invention is described herein for use in amplifiers employed in a cascaded two-stage D/A converter.
2. Description of the Prior Art
Susset U.S. Pat. No. 3,997,892 shows a two-stage cascaded D/A converter wherein the first stage includes a resistor-string DAC to produce a first segment voltage corresponding to a set of higher-order input bits. That voltage is directed through buffer amplifiers to the second stage converter which interpolates the selected first-stage segment voltage in accordance with the lower-order bits.
For such a converter to perform successfully at high resolutions, such as 16-bits, the inter-stage buffer amplifiers must meet extremely stringent specifications. It has been found that such specifications cannot be achieved using known techniques.